1. Field of the Invention
This invention relates to an image sensing apparatus so adapted that optimum exposure control is carried out with regard to any range of video signals obtained by imaging.
2. Description of the Related Art
The recent progress made in the field of video equipment such as video cameras has been remarkable and such equipment has been provided with greater functionality and capability. In order for such image sensing apparatus to be able to perform optimum photography at all times under a large variety of photographic conditions, exposure control is very important. For example, it is difficult for the photographer to optimally adjust the exposure of the subject at all times while maintaining the balance between the exposure of the subject and the exposure of the surroundings of the subject. Often the image resulting from photography is too bright or too dark (what is caled saturation or noise (darkness)).
Exposure control in a conventional image sensing apparatus will now be described.
FIG. 4 is a block diagram showing an example of the construction of an image sensing apparatus such as a video camera which exemplifies the prior art.
The image sensing apparatus of FIG. 4 includes a lens group 1 for imaging a subject, and a diaphragm mechanism 2 such as an iris 2 having a diaphragm vane structure for controlling the amount of incident light, or liquid-crystal means for controlling the amount of transmitted light.)
The image sensing apparatus further includes an image sensing device 3 such as a CCD for photoelectrically converting the incident light, an iris encoder (IE) 4 serving as iris detecting means using a Hall device or the like to detect the state of the diaphragm mechanism, a drive motor 5 such as an iris galvanometer (IG meter) for driving the diaphragm mechanism, a circuit 6 for controlling the image sensing device 3, reading out the image signal resulting from the photoelectric conversion and controlling a so-called electronic shutter function, namely a function which controls signal storage time. A sample-and-hold circuit 7 samples and holds the signal resulting from the photoelectric conversion performed by the image sensing device 3. An automatic gain control circuit (referred to as an “AGC” circuit below) 8 varies gain for electrically amplifying the image signal outputted by the sample-and-hold circuit 7.
A camera-signal processing circuit 9a subjects the signal from the AGC circuit 8 to processing such as a gamma correction, color separation and color-difference matrix operation and the adds on a synchronizing signal to produce a standard TV signal. This circuit applies processing to a signal that is in analog form.
A video tape recorder (referred to as a “VTR” below) 10 records the video signal from the processing circuit 9a on tape. An electronic viewfinder (referred to as an “EVF” below) 11 is for monitoring the video being shot. An AE detector circuit 12 detects the state of exposure of a subject based upon the output video signal from the AGC circuit 8. This circuit comprises an integrating circuit which integrates the luminance level of the video signal over a prescribed period of time.
On the basis of the output signal from the AE detector circuit 12, the exposure control circuit 13 controls the diaphragm mechanism 2, the shutter speed of the electronic shutter controlled by the control circuit 6 and the gain of the AGC circuit 8.
An iris drive circuit 14 actuates a motor to drive the diaphragm mechanism 2 in dependence upon the output of the exposure control circuit 13, an exposure setting key 15 for setting exposure at the time of manual exposure control, and an exposure control automatic/manual selection key 16 for selecting the method of exposure control.
Operation within the exposure control circuit 13 is as follows: A mean luminance level outputted by the AE detector circuit 12 is supplied to an automatic exposure control circuit 13d, which is for automatic iris control. The circuit 13d compares the level signal with a reference signal and controls an AGC control circuit 13a, an electronic shutter control circuit 13b for varying storage time of the image sensing device and an iris control circuit 13c, thereby controlling the operating states of the AGC, electronic shutter and iris in such a manner that the luminance level of the video signal takes on a predetermined value.
Further, a manual exposure control circuit 13e performs manual control of the iris. Specifically, in response to operation of the exposure setting key 15, iris f-stop is read out of a look-up table (LUT) 13h and supplied to a comparator circuit 13g, which compares this value with f-stop information from the iris encoder 4. The comparator circuit 13g is so adapted as to output information conforming to the difference between the value obtained by operating the exposure setting key 15 and the f-stop from the iris encoder 4.
The iris control information outputted by the comparator circuit 13g is subjected to a prescribed exposure correction or a correction of diaphragm characteristic by a correction arithmetic circuit 13f, after which the corrected information is supplied to the iris control circuit 13c to drive the iris 2 via the iris drive motor 6.
As a result, the iris 2 is driven until the input value to the comparator circuit 13g, namely the f-stop of the iris, becomes equal to the f-stop set by the exposure setting key 15. This makes possible manual control of the iris.
Further, if switched to an A contact in response to operation of the exposure control automatic/manual selection key 16, a switch A/MSW selects the automatic exposure control circuit 13d. If switched to an M contact by the exposure control automatic/manual selection key 16, the switch A/MSW selects the manual exposure control circuit 13e. 
In order to make it possible to obtain optimum video at various locations and under a variety of conditions by simple photography with the image sensing apparatus thus constructed, a change in exposure due to a change in the subject is detected from the video signal by means of the AE detector circuit 12, and, on the basis of the output signal from the AE detector circuit 12, the exposure control circuit 13 selects exposure control parameters for the diaphragm mechanism 2, the electronic shutter which controls the storage time of the image sensing device 3 and the gain of the AGC circuit 8 and decides the amount of correction to be applied to each of these parameters. As a result, control is carried out so as to obtain stable, optimum exposure at all times.
Thus, optimum exposure control can be performed automatically without any difficulty on the part of the photographer. Furthermore, the AE detector circuit 12 makes optimum photography possible by controlling photometric distribution based upon setting of the detection region or detection position of the video signal for exposure control.
For example, it is possible to perform so-called mean photometry, in which the entire video region is detected and exposure is controlled in such a manner that this detection signal takes on a constant level, or center-emphasizing photometry, in which only the central portion of the video region is detected and exposure is controlled in such a manner that this detection signal takes on a constant level.
Further, the AE detector circuit can perform exposure control based upon photometry that combines mean photometry and center-emphasizing photometry. This can be accomplished by weighting the detection data of the entire video region and the detection data of the region where center is emphasized, and performing exposure control based upon detection data obtained by adding these data at a fixed ratio. This compensates for the drawbacks of the individual photometric methods and provides even better exposure control.
Further, finer exposure control can be achieved by subdividing a scene into areas, detecting video in each area and limiting the areas of detection data used in exposure control or changing weighting.
However, even the photometric methods described above do not always provide a state of exposure control that is intended by the photographer.
For example, when a person is photographed in the presence of backlighting, the exposure of the person is affected by the brilliant background. This results noise (darkness). Conversely, when a person is photographed under strong frontlighting, as when the person is illuminated by a spotlight, the state of exposure results saturation.
Manual exposure correcting means have been proposed heretofore to allow the photographer to set exposure in order to deal with the photographic conditions mentioned above. So-called manual iris means will be described as one example of such manual exposure correcting means. The manual iris means maintains the state of the diaphragm mechanism 2 in a state set by the photographer, regardless of the detection signal from the AE detector circuit 12.
Specifically, the photographer selects automatic or manual for the exposure control method by the exposure control automatic/manual selection switch 16. If the manual exposure control method has been selected (i.e., if the A/MSW switch has been set to the M contact in FIG. 4), the control means within the exposure control circuit 13 is switched over to the manual exposure control circuit 13e. 
The manual exposure control circuit 13e has the look-up table (LUT) 13h and the comparator circuit 13g which compares the LUT data with the detection signal from the iris encoder 4 that detects the opening degree of the iris. As shown at {circle around (1)} in FIG. 6, typical diaphragm states represented by F values from a fully open iris to a fully closed iris are set in the LUT 13h, and values from the iris encoder 4 corresponding to these F values are also set in the LUT.
While monitoring the screen of the EVF 11, the photographer uses the exposure setting key 15 to select the desired brightness of the subject. As a result, data to be read out of the LUT are selected in dependence upon the setting signal from the exposure setting key 15, and the comparator circuit 13g compares these data with the output signal of the iris encoder 4.
If there is a difference between the signals compared by the comparator circuit 13g, an amount of correction is calculated and the iris is controlled until the value from the iris encoder 4 becomes equal to the data read out of the LUT 13h. 
Thus, it is possible for the exposure of the subject to be corrected to the brightness intended by the photographer. In this operation, the control values of the AGC and electronic shutter are set to fixed values that have been stored in the LUT 13h in advance or to control values which prevailed when the exposure control method was switched from automatic to manual. However, in a case where a set value is selected by the exposure setting key 15 in the same manner as the iris, a fine exposure setting in conformity with the photographic conditions is carried out.
However, regardless of the fact that the exposure correction is performed by the photographer using the manual exposure control means while the photographer observes the EVF 11 (FIG. 4) serving as monitoring means, generally the EVF 11 is made small enough to be observed by one eye. As a consequence, the size of the screen is small. Furthermore, in an EVF using liquid-crystal display means for the purpose of providing a color display, the dynamic range is insufficient. Thus, the small size of the screen and the characteristics of the liquid-crystal display means make it difficult to accurately discern the state of exposure of the subject. As a result, it is difficult to set the optimum exposure and a disparity can develop between the state of exposure intended by the photographer and the state of exposure actually set. Further, even if subject exposure intended by the photographer is achieved, the periphery of the subject results saturation or darkness, and the resulting video may have poor balance in terms of the exposure of the scene overall.
Furthermore, since complicated key operation is necessary, the photographer is required to have some degree of photographic experience. A photographer with little photographic experience cannot master operation with ease.